Method of producing zinc from chloride solutions which contain chiefly iron, copper and zinc

ABSTRACT

Selective extraction of zinc from chloride solutions which contain iron, copper and zinc, where the zinc is extracted in organic phase by means of tri-n-butylphosphate in an organic solvent. The extraction is carried out on chloride solutions in which the lead contents is possibly lowered and where the copper contents is maximum 50 grams per liter higher copper concentration being reduced for instance by electrolysis in diaphragm cell. The zinc containing organic phase is washed, and zinc back-extracted and conducted to a diaphragm cell for electrolysis of zinc, the used electrolyte being returned to the back-extraction step. The raffinate from the zinc extraction is used for repeated leaching of metals after preceding reoxidation.

The present application is a continuation-in-part of our earlier filedapplication Ser. No. 255,475 filed Apr. 20, 1981, now abandoned which inturn is a continuation of the earlier filed parent application Ser. No.137,569 filed Apr. 7, 1980, now abandoned.

The invention relates to a method of producing zinc by liquid-liquidextraction from chloride solutions which contain iron, copper and zincand subsequent electrolysis of zinc from the chloride solution andparticularly at removing zinc from those solutions which are formed whencomplex copper-zinc-lead ores are leached with iron chloride inprocesses which are known per se. The extraction is carried out on thatsolution which is formed when the contents of copper possibly has beenlowered in known way.

By the method of the invention the lead is, if necessary, precipitatedas lead chloride in known way by crystallization at lowered temperature,whereupon the extraction of zinc is carried out on the residualsolution, possibly after removal or reduction of the copper contents.The zinc is extracted into the organic phase by means oftri-n-butyl-phosphate (TBP) diluted in an organic solvent, whereupon theorganic phase and the lye is separated. The organic phase is before theback-extraction washed with a portion of the product solution in orderto remove the chief part of co-extracted copper and iron. Used zincchloride electrolyte from the subsequent zinc chloride electrolysis isutilized for the back extraction, and the resulting product solution iscleaned for iron and copper and other impurities by known methods andpossibly supplied with an organic glazing means of for instancepolyacryl type. The zinc chloride electrolysis is preferably carried outin a diaphragm cell. As diaphragm there can be used different qualitiesof artificial fiber cloth as for instance polypropylene or teflon clothand there is preferably used DS-anodes and aluminum cathodes. Thatchlorine which is developed on the anodes is returned to the process,while the zinc is precipitated on the cathodes and removed regularly.

In order to obtain a selective zinc chloride extraction fromiron-copper-zinc chloride lye which is formed after the possiblereduction of the contents of lead, it is important that the copperconcentration does not exceed a certain value, and the inventors havefound that the copper concentration should not exceed 50 grams perliter. If the copper concentration in the lye is to be reduced, a partof the copper can be removed in known way, for instance by cementation,electrolysis or hydrogen reduction. The reduction of the copper contentsis carried out until the copper concentration has reached the desiredvalue. The feed solution to the zinc extraction step can for instance betaken from the catholyte solution in the copper electrolysis. The zincconcentration in this solution is unaltered.

EXAMPLE I

A bulk concentrate containing iron, copper, zinc and lead with 11% Cu,25% Fe, 18.4% Zn and 3.7% Pb was leached with a chloride solutioncontaining iron, copper, zinc and lead, and the lead concentration wasreduced by crystallization in known way. The lye after the leadcrystallization contained 46 grams per liter Cu, 180 grams per liter Fe,92 grams per liter Zn and 5 grams per liter Pb. In order to reduce thecopper concentration to about 25 grams per liter Cu, the solution wasfirst conducted to a diaphragm cell in which it was electrolyzed untilthe desired concentration was obtained. From the catholyte compartmentof the electrolysis cell the solution with the reduced copper contentswas conducted to the continuously operating extraction equipment inwhich the zinc chloride was extracted with TBP diluted by Solvesso 150.The organic phase which contained 22.4 grams per liter Zn was firstwashed with a part of the product solution from the back extractionstep, whereupon it was back extracted with used zinc chlorideelectrolyte from the zinc electrolysis. This contained 25 grams perliter Zn. The concentration of zinc was hereby increased to 60 grams perliter (g/l hereinafter). Some iron, copper and other impurities was alsoback extracted, and impurities were as mentioned above removed from theso formed product solution to a content which can be tolerated by theknown methods for zinc electrolysis. The product solution was suppliedwith an organic glazing means in a concentration of 5 parts per million(ppm) and electrolyzed in a diaphragm cell with DS-anodes and aluminumcathodes. The diaphragm consisted of a polypropylene cloth. The ingoingelectrolyte (product solution) contained 60 g/l Zn and 4 g/l HCl, whilethe outgoing contained 25 g/l Zn and 0.5 g/l HCl. The outgoingelectrolyte was as mentioned above returned to the back extraction step.The raffinate from the zinc extraction can be used for another leachingof metals after being reoxidized in known way.

FIG. 1 illustrates a preferred closed loop process in a secondembodiment of the present invention described in Example II below.

FIG. 2 is a flow diagram of a preferred system for organic extraction ofzinc in Example II.

FIG. 3 is a flow diagram of a preferred system for purifying the zincrich aqueous medium before zinc electrolysis in Example II.

EXAMPLE II

A sulfidic bulk concentrate containing iron, copper, zinc, and lead hasbeen leached with chloride leach liquor by the known red-ox principle.The active oxidized species in the liquor are the ferric ions, and themetal sulfides are attacked according to the equations:

    3FeCl.sub.3 +CuFeS.sub.2 =CuCl+4FeCl.sub.2 +S°      (1)

    ZnS+2FeCl.sub.3 =ZnCl.sub.2 +FeCl.sub.2 =S°         (2)

    PbS+2FeCl.sub.3 =PbCl.sub.2 +2FeCl.sub.2 +S°        (3)

A small amount of the elementary sulfur formed by reactions (1) through(3) is oxidized further to sulfate:

    6FeCl.sub.3 +4H.sub.2 O+S°=6FeCl.sub.2 +6HCl+H.sub.2 SO.sub.4 (4)

Pyrite (FeS₂) in the concentrate is not attacked by the ferric ions.

The composition of the leach liquor going into the leach system is (allanalyses in g/l):

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot   Cu.sup.2+                                                                Cu.sup.30                                                                          Zn   Pb   HCl  SO.sub.4.sup.2-                ______________________________________                                        150  135    15      25  25 0    48.1 21.1 4.0  5.0                            ______________________________________                                    

To this leach liquor was added a sulfide concentrate with analysis shownbelow (weight percent):

    ______________________________________                                        Zn       Cu             Pb    S                                               ______________________________________                                        29.0     7.7            8.1   40                                              ______________________________________                                    

The leaching was performed in an apparatus described in a copendingapplication. Referring to FIG. 1, the leach liquor to the leachingsystem is split in two parallel streams. The main (primary) stream 10 isused in a substoichiometric leach with respect to available sulfides inthe concentrate, and the secondary stream 12 is used in asuperstoichiometric leach on the residue from the primary leach.

With this particular concentrate, the primary leach operation was doneat 14 in two tanks, each of 30 liter capacity and made ofglassfibre-reinforced polyester (GRP), connected in series. Each tankwas equipped with a stirrer of the propeller type, and baffles, to avoidsettling of solids in the tanks during operation.

The concentrate was fed to the first leach tank by means of a vibratingscrew-feeder and at a rate that gave an amount of leach liquorsubstoichiometrically to the available metal sulfides in theconcentrate.

The ratio of concentrate to leach liquor was 360 grams concentrate perliter of leach liquor added.

The residence time in each tank was one hour, and the temperature 105°C. The slurry from tank No. 1 was fed into tank No. 2 via an overflowconnection.

The slurry out of tank No. 2 was pumped to a conventional thickener (notshown) made of rubber-coated steel. The underflow from the thickener,containing the solids, was sent to the secondary leach at 16 for anessentially complete extraction of the remaining metal values.

The remaining solids from the secondary leach, composed mainly of gangueand sulfur, were taken out in a filter press 18 made of polypropyleneand operated at 105° C. The fresh leach liquor addition to the secondarystage was, with this particular concentrate, added at a rate thatensured a red/ox potential in the secondary leach tank of 500 mV asmeasured with the Pt/calomel couple. This ensures essentially completeextraction of the remaining metals in the residue from the primary leachstage.

The leach residue at 20 was washed countercurrently with the HClsolution resulting from the zinc chloride purification (see later).

The overflow from the thickener, containing the metal values aschlorides, and essentially reduced (i.e. no ferric present, and onlytraces of cupric) was pumped to the lead chloride crystallization stage22. The analysis of the liquid overflow from the thickener, beforeentering the crystallizer at 22 was (analysis in g/l):

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot                                                                         Cu.sup.2+                                                                           Cu.sup.30                                                                          Zn   Pb  HCl  SO.sub.4.sup.2-              ______________________________________                                        153  0      153    38   5     33   104  30  19.5 11.4                         ______________________________________                                    

The crystallizer itself was of the known vacuum-flash type. Thetemperature of the entering liquid is lowered from the initial 105° C.down to 10° C. by reducing the pressure above the liquid and boiling offwater. The lead chloride was collected in a fluidized bed of circulatingcrystals.

The materials of construction of the crystallizer are titanium and glassin all parts in contact with the liquid. Fluidization of lead chloridecrystals was achieved by means of an internal circulation of liquid by atitanium propeller.

The lead chloride particles formed in the circulating liquid grow to adefinite size and drop out of the bed and down to the bottom, where theyare intermittently removed via a barometric leg.

The liquid leaving the crystallizer has the following composition(analysis in g/l):

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot   Cu.sup.2+                                                                Cu.sup.30                                                                          Zn   Pb   HCl  SO.sub.4.sup.2-                ______________________________________                                        186.3                                                                              0      186.3   46.4 6.0                                                                             40.4 126.5                                                                              12.0 23.7 13.9                           ______________________________________                                    

The increases in iron, copper, zinc, hydrochloric acid, and sulfateconcentrations are due to the water evaporation in the vacuum flashingof the hot incoming liquid.

The main part of the lead carried into the system by the concentrate isremoved as lead chloride in this step. The balance is precipitated aslead jarosite in the oxidation step.

The produced lead chloride is washed on a filter at 24 and the usedwash-water added to the process stream. This decreases theconcentrations somewhat (analysis in g/l):

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot   Cu.sup.2+                                                                Cu.sup.30                                                                          Zn   Pb   HCl  SO.sub.4.sup.2-                ______________________________________                                        185.2                                                                              0      185.2   46.2 6.0                                                                             40.2 125.8                                                                              12.0 23.5 13.8                           ______________________________________                                    

This liquid is fed to the cathode compartment 26 in the known copperelectrowinning diaphragm cell 28 and is totally reduced by the influenceof the current flowing in the cell. The necessary amount of copper tobalance out the amount added in the concentrate feed is deposited on thecathodes as a powder, which falls off and sinks to the bottom of thecell. The bottom of the cell is conical, to achieve good collection ofthe produced copper powder. The powder is intermittently removed fromthe bottom through an air-operated valve.

Materials of construction in the copper electro-winning cell 28 are GRPand rubber in all parts that are in contact with the liquid. Thecathodes were made of titanium, and the anodes were of the so-called DSAtype (dimensionally stable anodes, titanium with a noble metal oxidelayer).

The electrolysis temperature was in the range of 40°-90° C.

The catholyte in the copper electrowinning cell is used as feed to thezinc solvent extraction at 30. The produced raffinate is returned to theanode compartment 32 of the copper EW cell 28. The importance and greatadvantage of using the catholyte as feed to the SX system lies in thevery large difference in the distribution coefficients for zinc chlorideand iron (III) chloride. The table below gives measured values of thesecoefficients in the actual chloride solution, with the used tri-n-butylphosphate extractant:

    D.sub.Zn =0.25

    D.sub.Fe(III) =10

    D.sub.Fe(II) =0.012

    D.sub.Cu(I) =0.082

    D.sub.Cu(II) =0.008

As one sees, the importance of keeping oxidized species (Cu (II) and Fe(III)) at as low as possible values cannot be stressed enough. Theinfluence of copper (II) is caused by the equilibrium between theoxidized and reduced species in the solution:

    Cu.sup.2+ +Fe.sup.2+ =Cu.sup.+ +Fe.sup.3+                  (5)

This equilibrium will be disturbed when contacting a solution withessentially no iron (III), but measurable amounts of copper (II), withan extractant with a strong affinity for iron (III), as the case is withTBP: ##STR1##

We therefore run the electrolysis at 28 at a red/ox potential (less than300 mV) that assures us that no ferric is present in the catholyte, andthat all the copper is in the cuprous state. This catholyte, with ananalysis as given below (analysis in g/l):

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot   Cu.sup.2+                                                                Cu.sup.30                                                                          Zn   Pb   HCl  SO.sub.4.sup.2-                ______________________________________                                        182.7                                                                              0      182.7   25.9   0                                                                             25.9 124.7                                                                              11.9 23.4 13.7                           ______________________________________                                    

The other important advantage of using the catholyte as fed to the SXstage is that copper is at its lowest concentration in this liquid, andcoextraction of copper is therefore minimized.

The drop in copper content reflects the production of the same amount ofcopper that was dissolved from the concentrate in the leachingoperation. The small drop in the concentration of the other elementspresent is caused by washing of the produced copper powder at 26 withwater, and the returning of the wash-water to the process solution.

FIG. 2 illustrates a preferred system of TBP solvent extraction of zincbut it will be understood that any conventional extraction and scrubsystem may be employed. The catholyte from 26 is fed to themixer-settlers 34 of solvent extraction at 30 where it is contacted withthe organic extractant tri-n-butyl-phosphate (TBP). The extractant wasdiluted with a commercial available diluent, Exxon Solvesso 150. Theconcentration of the extractant TBP in the diluent was 40 volumepercent. The feed solution was cooled to 30° C. by passing it through atitanium heat exchanger (not shown) before it entered themixer-settlers.

The mixer-settlers 36 were of the known pump-mix type, made inglassfibre reinforced polyester. The dimensions of the mixer (L×W×H)were 200×200×200 mm, and the adjoining settler (L×W×H) 500×200×200 mm.The mixer-settlers were equipped with lids. A guiding plate (not shown)was located at the inlet of the settler to lower the linear velocity ofthe emulsion from mixer and inject it at the interphase of the settler.The impellers, with 6 radial blades on both sides, were made inpolypropylene. They had a diameter of 100 mm and were operated at 425rpm. Connections and piping were mostly made in polypropylene, but somepolyethylene and a special brand of rubber hose were also used.

Piston type dosage pump were used for metering all the aqueous flows,except for the two recirculating aqueous scrub phases were centrifugalpumps were used. A pneumatic metering pump completely in teflon andglass for all parts in touch with liquid to be pumped, was employed forthe organic phase.

Because of the pulsation created by the piston pumps, these flow rateswere controlled simply by measuring the amount of liquid within acertain time. The flow rates in the circuits belonging to thecentrifugal pumps were checked by rotameters.

The extraction section consists of three mixer-settler stages 36. Afraction of the feed liquor is fed at 38 directly to the last extractionstage, to avoid crud formation due to low acidity, which is caused bythe tendency of TBP to extract hydrochloric acid.

The iron scrub at 40 consists of two stages 42, to scrub out most of thecoextracted iron. As ferric is extracted to a much higher extent thanferrous, it is essential that both the extraction and the iron-scrubstages are kept inert, and at a low red/ox potential in the aqueousphases.

The copper-scrub section 44 has one stage only. The section is operatedunder oxidizing conditions, using air as oxidizing agent in the aqueousphase. The reason is that cuprous is more readily extracted by TBP thatcupric. Coextracted copper is thus scrubbed out of the organic phaserather effectively by air oxidized aqueous phase. As indicated on theflowsheet in FIG. 2, air is injected only into the aqueous phase. Thisis performed in a surge/pump tank for the recirculating aqueous phase.

The strip section 46 consists of four stages.

The storage/surge tank for organic phase is not shown on the flowsheet.The tank was located between the strip and the extraction sections 46and 34, respectively. Further, a tank (not shown) was installed in theraffinate stream to prevent break-through of organic phase to the copperelectrowinning cell in case of improper interphase level in the settlerof the third extraction stage 36. The tank was supplied with a baffle tolet the underflow only to the copper cell.

The preferred flow of liquids in the extraction, scrub and strip stagesis illustrated in FIG. 2. As there shown, the TBP is continuouslycirculated in a closed loop as shown by the heavy black line 48 of FIG.2. Preferably the extraction at 34 is effected by countercurrent flow ofTBP with respect to the catholyte from 26 and if desired anyconventional solvent extraction may be employed in place of themixer-settlers 36.

The aqueous medium used in the stripping and scrub stages is preferablythe spent electrolyte from the zinc electrolysis (later described) andthe flow of this aqueous medium is shown in the lighter line 50 of FIG.2. Stripping of the TBP at 46 is carried out in conventional apparatusand preferably mixer-settlers 36 are employed in which the aqueousmedium and TBP flow countercurrently. A portion (approximately 10%) ofthe aqueous medium rich in zinc is bled off at 51 for use in the ironand copper scrub steps at 40 and 44 respectively. Scrubbing is carriedout in conventional apparatus such as mixer-settlers 36 and the aqueousscrub solution from 40 and 44 is fed back into the closed loop of themain circuit after the copper electrowinning cell 28 for oxidation andprecipitation of iron. The aqueous medium rich in zinc from the stripstage 46 if necessary is pumped to a solution purification at 52(FIG. 1) to remove any remaining iron and copper and other impuritiesprior to zinc electrolysis.

The raffinate from the extraction step 34 is returned to the copperelectrowinning cell, and purged into the anode compartment 32 of thecell 28.

The analysis of the raffinate to 32 is as follows (g/l):

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot   Cu.sup.2+                                                                Cu.sup.30                                                                          Zn   Pb   HCl  SO.sub.4.sup.2-                ______________________________________                                        188.2                                                                              0      188.2   23.3   0                                                                             23.3 18.8 13.5 24.6 15.5                           ______________________________________                                    

The necessary amount of ZnCl₂ in the catholyte from 26 is extracted at34 into the organic phase together with small amounts of copper andiron. The phase volume ratio of aqueous feed flow/organic flow was0.146.

The concentration profiles over the extraction-, scrub- and strip stagesin the SX sequence shown in FIG. 2 were (all values in g/l):

    ______________________________________                                        Zinc         Copper        Iron                                                     Or-    Aque-                                                            Stage ganic  ous     Organic                                                                              Aqueous                                                                              Organic                                                                              Aqueous                             ______________________________________                                        Extrac-                                                                             9.80   13.8    1.80   21.8   2.10   175.0                               tion 3                                                                        Extrac-                                                                             15.40  57.5    2.18   25.3   2.40   180.2                               tion 2                                                                        Extrac-                                                                             18.00  101.1   2.60   25.9   2.66   182.6                               tion 1                                                                        Iron  18.40  83      1.72   36.0   0.96   80.1                                scrub 2                                                                       Iron  19.20  84      1.60   34.5   0.74   80.2                                scrub 1                                                                       Copper                                                                              19.40  83      0.24   89.0   0.70   13.7                                scrub                                                                         Strip 4                                                                             11.0   75.0    0.0031 0.53   0.0302 2.5                                 Strip 3                                                                             6.14   49.1    0.0016 0.013  0.0060 0.092                               Strip 2                                                                             3.36   33.6    0.00084                                                                              <0.002 0.0018 0.010                               Strip 1                                                                             1.80   25.6    0.0068 <0.002 0.001  <0.005                              Strip in                                                                            --     22.0    --     0      --     --                                  Extrac-                                                                             --     124.7   --     25.9   --     182.7                               tion                                                                          feed in                                                                       ______________________________________                                    

The iron scrub at 40 was 7.6 volume percent of strip in, and the copperscrub at 44 was 3.6 volume percent of strip in.

The strip solution out of mixer-settler 46 No. 4 (strip 4) wasapproximately equal to the amount of feed solution to the extractionstep.

This solution from 46 was submitted to a purification treatment at 52 todecrease the concentrations of iron and copper to acceptable levels. Theprocedure for the iron removal is shown schematically in FIG. 3. A partof the solution from strip stage 4 (approximately 10%) is treated withlime at 54 to achieve a precipitation of the zinc in the solution aszinc hydroxide.

    CaO+ZnCl.sub.2 +H.sub.2 O=Zn(OH).sub.2 (s)+CaCl.sub.2      (7)

The other metals contained in this stream is similarly precipitated ashydroxides. The pH of the resulting solution is around 7.

The produced zinc hydroxide, (eq. 7), also containing the copper andiron precipitate, was separated in conventional manner at 56 from theresulting calcium chloride solution and used as a base to treat the mainamount of strip solution at 58.

The calcium chloride solution from the zinc hydroxide precipitation wastreated with sulfuric acid at 60 to generate HCl and deposit calcium asgypsum:

    CaCl.sub.2 +H.sub.2 SO.sub.4 +2H.sub.2 O=CaSO.sub.4 ·2H.sub.2 O(s)+2HCl                                                 (8)

After filtering of the gypsum at 62, the HCl solution was concentratedat 64 and the resulting HCl solution was utilized in the main leachingcircuit for washing purposes, to maintain the chloride and red/oxbalance in the process as desired.

The zinc hydroxide was added to the main strip solution at 58 to adjustthe pH in this solution to 2-2.5, and at the same time air was injectedinto the solution to oxidize ferrous ions to ferric, and precipitateferric hydroxide:

    Zn(OH).sub.2 (s)+FeCl.sub.2 +1/2H.sub.2 O+1/4O.sub.2 =ZnCl.sub.2 +Fe(OH).sub.3 (s)                                         (9)

The precipitation reaction was done in a stirred GPR vessel at 80° C.and the resulting iron precipitate was taken out by filtration at 66.

The filtrate was pumped to a second GRP tank 68 equipped with stirrer,and here zinc dust was added to precipitate copper:

    Zn(s)+CuCl.sub.2 =Cu(s)+ZnCl.sub.2                         (10)

Copper cement was removed in a small rubber-lined steel thickener. Thepurified solution was then re-adjusted to pH 1 at 70 before entering thezinc electrowinning cell 72 (FIG. 1). The analysis of the feed solutionto the zinc cell 72 is (g/l):

    ______________________________________                                        Zn    HCl                                                                     ______________________________________                                        78    3.5         No iron, copper, lead or sulfate.                           ______________________________________                                    

The electrolysis was done in a conventional diaphragm cell 72 made fromGRP with aluminum cathodes (0.5 m² each) and DSA. Diaphragms were madeof polypropylene fabric. In this cell, zinc chloride was decomposed:

    ZnCl.sub.2 =Zn(s)+Cl.sub.2 (g)                             (11)

Zinc is deposited on the aluminum cathodes as a solid layer, and thecathodes were stripped every 24 hours. The chlorine gas evolved on theDSA's were returned to the oxidation stage at 74 in the main process toachieve chlorine as well as red/ox balance.

The cathodic current density used in cell 72 was 400 Am⁻², and theanodic current density somewhat higher.

Anolyte from the zinc electrowinning cell, now depleted down to 20 g/lzinc, was returned to the strip stage 46 via a chlorine strip tower (notshown) to remove dissolved chlorine gas. In this tower, made ofpolypropylene and filled with ceramic saddles, and with a height of 4 mand a diameter of 20 cm, the dissolved chlorine was stripped withcompressed air. The resulting air/chlorine mixture was also returned tothe oxidation stage at 74 in the main hydrometallurgical circuit.

The liquor from the secondary leach stage 16, 18 and the anolyte fromthe copper electrowinning operation at 28 may be combined at 76 to enterthe air oxidation equipment 78. The analysis of this solution was (g/l):

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot   Cu.sup.2+                                                                Cu.sup.+                                                                           Zn   Pb   HCl  SO.sub.4.sup.2-                ______________________________________                                        155.2                                                                              58.3   96.9    25.2 8.0                                                                             17.2 48.4 21.6 14.0 9.5                            ______________________________________                                    

Equipment 78 consisted of a GRP tank with effective volume 60 liters,with air distribution in the bottom and an impeller-type air dispergeroperated at 550 rpm. The temperature was maintained at 80° C. in thistank by means of electric heating elements.

In the air oxidation stage, the iron dissolved in the leach stages isprecipitated, partially as sodium jarosite, partially as goethite, andexcess acid produced in the leaching of sulfur is used up. Sodium forthe jarosite precipitation is added as NaCl. The sulfur oxidized tosulfate in the leaching is also taken care of:

Jarosite formation and sulfate removal at 78:

    NaCl+3/4O.sub.2 +6FeCl.sub.2 +2H.sub.2 SO.sub.4 +3H.sub.2 O=NaFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 (s)+3FeCl.sub.3 +4HCl         (12)

A small part of the solubilized lead is precipitated as lead jarosite.

Excess acid removal:

    HCl+1/4O.sub.2 +FeCl.sub.2 =FeCl.sub.3 +1/2H.sub.2 O       (13)

Goethite precipitation:

    3FeCl.sub.2 +3/4O.sub.2 +1/2H.sub.2 O=FeO.OH(s)+2FeCl.sub.3 (14)

HCl and sulfate is consumed, and the red/ox potential partly restored.The jarosite/goethite precipitate amounts to 18 g/l solution. Thisprecipitate is taken out in a polypropylene filter press at 80.

The residue collected in the filter press was washed countercurrently inthree stages with water, and the washwater from the last stage added tothe main process stream.

The partially oxidized solution, with analysis (g/l) as given below,enters the chlorine oxidation stage at 82.

    ______________________________________                                        Fe.sub.tot                                                                         Fe.sup.3+                                                                            Fe.sup.2+                                                                            Cu.sub.tot   Cu.sup.2+                                                                Cu.sup.30                                                                          Zn   Pb   HCl  SO.sub.4.sup.2-                ______________________________________                                        150  63     87      25  25 0    48.4 21.6 4.0  5.0                            ______________________________________                                    

The chlorine oxidation stage 82 is made of a chlorination tower, withheight 1.5 m and 20 cm diameter. The tower is filled with ceramicsaddles. The partially oxidized solution with the above-mentionedcomposition was introduced at the top of this tower at a temperature of80° C., and the chlorine/air gas mixture at the bottom. In its passagedown the length of the tower, the solution was oxidized back to itsoriginal strength with the chlorine gas coming from the zincelectrowinning operation. The exhaust gas from the oxidation tower wasfree from chlorine and could be released to the atmosphere. The reasonfor this is that a residual concentration of 15-25 g/l of ferrous ionsis always maintained in the outgoing liquid from the chlorine oxidationtower. The oxidation reaction is:

    FeCl+1/2Cl.sub.2 (g)=FeCl.sub.3

The regenerated liquid is now again pumped to the leach section, thusmaking the process cycle a closed loop.

If desired, the liquor from secondary leaching 16, 18 which has not gonethrough the metal recovery loop may be fed directly to the chlorineoxidation stage at 84.

About 5 to 10 percent of leach liquor is preferably bled off at 86 forseparation of solid impurities as by filtration at 88. The filtrate isrecycled back to the main stream of leach liquor.

It will be understood that all changes and modifications of thepreferred embodiments of the invention herein chosen for the purpose ofillustration which do not depart from the spirit and scope of theinvention are intended to be covered.

What is claimed is:
 1. Method for producing zinc metal from chloride solutions containing iron, copper and zinc by liquid-liquid extraction and electrolysis which comprises the steps of:(a) forming a chloride solution containing iron, copper and zinc chlorides in which solution the iron chloride is essentially ferrous chloride and the copper chloride is essentially cuprous chloride, (b) contacting said chloride solution with tri-n-butylphosphate which selectively extracts and removes zinc chloride from the ferrous chloride and cuprous chloride in said solution, (c) separating zinc chloride from said tri-n-butylphosphate, and (d) feeding the separated zinc chloride in aqueous solution into an electrolytic cell in which zinc metal is deposited at the cathode by electrolysis and removed from the cell.
 2. The method of claim 1 which includes the step of limiting said chloride solution to a copper content of not more than about 50 grams per liter.
 3. The method of claim 1 in which the step of separating zinc chloride from said tri-n-butylphosphate is carried out by recycling used electrolyte from said cell into contact with the tri-n-butylphosphate to remove zinc chloride therefrom and thereafter feeding the electrolyte rich in zinc chloride into said electrolytic cell.
 4. The method of claim 1 in which the zinc chloride solution is contacted by tri-n-butylphosphate in an organic solvent.
 5. The method of claim 3 in which a portion of the electrolyte after separation of zinc chloride from said tri-n-butylphosphate is recycled for washing said tri-n-butylphosphate prior to the step of removing the zinc chloride therefrom by used electrolyte from said cell. 